Why are there so many kinds of whales and dolphins?

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Date: July-August 1991
From: BioScience(Vol. 41, Issue 7)
Publisher: Oxford University Press
Document Type: Article
Length: 1,070 words

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Although the cetaceans are a small group, compared with, say, beetles (thought to have millions of species) or even with some other mammalian orders, such as the rodents (with 1700 species), they include 75-80 species and are extremely diverse morphologically. They range from the generalized dolphins, such as the bottlenose dolphin, Tursiops truncatus, to highly derived forms such as the right whale dolphin, Lissodelphis borealis, which has no dorsal fin; from river dolphins weighing 20 or 30 kg to leviathans such as the sperm whale, Physeter macrocephalus, weighing more than 100,000 kg.

Even 75 species seems to be a lot for such mobile animals. After all, the oceans are continuous. Some dolphins have been recorded as traveling as much as 50-100 km per day. And we know that many whales and dolphins, like other large marine animals such as tunas and sharks, are almost worldwide in distribution. Some of these species are closely related to each other. How did they arise?

One model of speciation in isolation has to do with past climate fluctuations during which cold water may have pinched off the tropical Atlantic one or more times. At present, warm-temperate water flows around the Cape of Good Hope from east to west during the austral summer. This flow provides communication between the tropical Indo-Pacific and the tropical Atlantic. During the austral winter, water around the cape is too cold for tropical cetaceans, as it probably was during the Pleistocene or earlier cold periods. The Cape of Good Hope was probably embedded in very cold water year round. This cold water isolated the tropical Atlantic from...

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Gale Document Number: GALE|A10995649